Use absolute URL for fragment-links.js

We already do this in 404.html. This helps fixing
whatwg/html-build#33.

Author: annevk

link-fixup.js

@@ -7,6 +7,6 @@ window.addEventListener('DOMContentLoaded', function () {
     return;
 
   var script = document.createElement('script');
-  script.src = 'fragment-links.js';
+  script.src = '/multipage/fragment-links.js';
   document.body.appendChild(script);
 });